Resource partitioning among five species of waterfowl (Anas spp.) at an autumn migratory stopover: combining stable isotope and mercury biomarkers

The Saskatchewan River Delta (SRD) is North America’s largest inland delta and an important stopover site for waterfowl in the Central Flyway. However, little is known about their basic feeding ecology at this site and how species segregate or overlap in resource use. We used stable carbon (δ13C) and nitrogen (δ15N) isotopes and mercury concentrations ([Hg]) in liver tissue to trace use of local nutrient sources by five waterfowl species and tested for differences in diets among species, sexes, and age groups. Macrophytes were the dominant food source for Northern Pintail (Anas acuta L., 1758) and American Wigeon (Anas americana Gmelin, 1789) with median proportions of 0.86 and 0.98, respectively. There was also evidence of partitioning of resources, as Blue-winged Teal (Anas discors L., 1766) and Green-winged Teal (Anas carolinensis Gmelin, 1789) consumed invertebrates, as did a subset of Mallards (Anas platyrhynchos L., 1758), suggesting that these birds might minimize competition for resources during the short staging period in the SRD when waterfowl densities are high. Other isotopes or tracers, such as [Hg] that varied among sources (0.03–0.20 μg·g–1 dry mass) and waterfowl species (0.22–3.19 μg·g–1 dry mass), can be used for further refining dietary estimates.